Wafer-scale fabrication of graphene

Transcription

1 Wafer-scale fabrication of graphene Sten Vollebregt, MSc Delft University of Technology, Delft Institute of Mircosystems and Nanotechnology Delft University of Technology Challenge the future

2 Delft University of Technology Delft Institute of Microsystems and Nanoelectronics Class 100, 100 mm wafer size Full IC & MEMS processing line 2

3 Overview Graphene Properties Applications: chemical and Hall sensors Approaches of graphene manufacturing Towards wafer-scale fabrication Graphene layer transfer Integration aspects Conclusion 3

4 Graphene Single atom layer of graphite Theoretically known since 1947 First isolated in 2004 by Geim and Novoselov (Noble price 2010) First 2D material 4

5 Graphene properties Single-atom thickness: 0.34 nm Strongest material (> 1 Tpa), while extremely flexible Stainless steel 200 GPa Huge electron mobility (theoretically 200,000 cm K) Silicon 1400 cm 2 /Vs Optically transparent (2.3% absorption per layer) High thermal conductivity (>4800 W/mK) Copper 400 W/mK 2D: only surface, no bulk: extremely sensitive to interaction with surroundings Planar technology 5

6 Potential applications Field effect transistors Chemical sensors Hall sensors (magnetic field) Supercapacitors Interconnects Transparant electrodes Flexible electronics Thermal management Quantum dots Nanopores (DNA sequencing) Resistance standard 6

7 Graphene FET High mobility No bandgap! Bandgap engineering possible Nano-ribbons Bi-layer graphene Resulting bandgap low Worse: reduces mobility Other new 2D materials have intrinsic bandgap (e.g. MoS 2 ) Y.-M. Lin et al, Sciene vol. 327 p. 662 (2010) 7

8 Graphene for chemical sensors No bulk: sensitivity Low noise Adsorption of molecules onto surface changes resistance, or FET transfer characteristics Possible to detect single molecules Can be functionalized F. Schedin et al. Nature 6, p. 652 (2007) 8

9 Graphene for Hall sensors Xu et al. Scientific Reports 3, 1207 (2013) High mobility Large Hall sensitivity Thin material Low noise Simple design Highly linear 1500 V/AT sensitivity demonstrated (Si 100 V/AT) 9

10 Graphene market predictions (transparent conductive films) March,

11 Graphene fabrication 11

12 Industrial up scaling? 12

13 Alternatives? Exfoliation using tape Exfoliation using ultrasonic Highest quality, flakes Never fit for industry SiC: ablation High quality, suspension Not suitable for wafer fabrication Chemical vapour deposition (CVD) 13

14 Epitaxial growth from SiC By heating up crystalline (4H or 6H) SiC to ~1500 C in high vacuum Si ablates, leaving C Large area (cm 2 ) High quality On top of semiconductor layer Extremely expensive substrate (2 wafer: ~$ 1000) No 8 or 12 substrates available? 14

15 Chemical vapour deposition Mono-layer: Cu Multi-layer: Ni, Co 15

16 Back gate voltage (V) High quality CVD graphene Group at TU Delft demonstrated CVD graphene on Cu films with 1 µm ballistic conduction at 4K by transverse magnetic focusing Mobility up to 20,000 cm K, mm-size graphene flakes (in press) 2 n B ~ e L Vgate Magnetic field (T) V/I bias (kω) Shou-En Zhu, Victor Calado, Lieven Vandersypen, Guido Janssen. Graphene 2013, Bilbao, Spain 16

17 Direct growth on wafers Scalability of thin metal foils to > 4 wafer size poor Direct growth on wafers required: catalyst on oxidized wafer First 300 mm tool installed in AIST, Japan 17

18 Analysing graphene 3.5 2D 3.0 Normalized intensity (a.u.) D G Raman shift (cm -1 ) Difficult to see graphene, spectroscopy techniques required 18

19 Transfer process Growth on wafers is scaling up fast However, layer still has to be transferred 19

20 Transfer results Wafer scale transfer still problematic Mobility low, although material is of good quality: polymer residues after transfer Tao et al. J. Phys. Chem. C 116, (2012) 20

21 Alternative transfer methods? Circumvent use of polymer? Etching away Cu to release graphene problematic for large wafers No etching: peal-off graphene film by promoting adhesion to target? Demonstrated for multi-layer graphene using PDMS Wafer-to-wafer direct transfer? Graphene layer transfer biggest challenge in graphene wafer-scale manufacturing 21

22 Watanabe et al, Diam. Relat. Mater. 24 (2012) Graphene integration Graphene likely first a back-end material Contamination issues Low resistance contact formation Au Ti Layer quality after transfer is key For Hall sensors & transistors high mobility required For chemical sensors quality demands less strict (but need functionalization) Most demonstrators single-shot devices: manufacturability hardly investigated. 22

23 Graphene: the next carbon nanotubes? After 20+ years or research on carbon nanotubes (CNT) still hardly any commercial application Properties of CNT very similar to graphene Fabrication issues holding back application of CNT: overhyped? Could the same happen to graphene? Advantage graphene: planar Investigation of manufacturability and scalability Use graphene where it has the advantage: sensors, transparent electrodes, flexible electronics, etc. Graphene FET? Maybe another 2D material will do better (MoS 2 ) 23

24 Conclusion Graphene is attractive material for sensors, transparent and flexible electronics Industrial production material holding back implementation CVD graphene most attractive method Scalable to 30 cm wafers Quality as good as exfoliated graphene Wafer transfer key: no transfer no practical use Manufacturability and uniformity have to be studied in detail 24